the Calculation of Flange Dimensions for a Pressure Vessel

By definition, a pressure vessel is a closed container that stores or processes oil, gas, and chemicals at a lower or higher pressurized condition than outside pressure conditions. For pressure vessels, flanges play a crucial role in ensuring effective sealing while allowing people’s entry to perform maintenance works.

Here, the 2197 pressure vessel design follows the ASME Section VIII Div 1 which also guides the flange design. Still, some may require incorporating ANSI/ASME B16.5 to incorporate a required level of precision. Even though experts at Yanhao use the latest software to design the flanges, understanding the manual calculation of flange dimensions for a pressure vessel is always fruitful to gain a solid basis. Let’s start.

Understanding of a Sealing Flange for Pressure Vessel

The stakeholders of Chinese flange manufacturing know that each pressure vessel requires substantially high-quality sealing flanges. From high fatigue resistance to corrosion resistance, all these resistive characteristics are essential for the seal flange of a pressure vessel. 

In particular, sealing flanges are used in the openings of pressurized containers. These specialized Chinese sealing flanges are also used in joining two or multiple vessels while these are also used to adjoin in and out piping, valves, regulators, etc.

Types of Sealing Flanges Used in Pressurized Containers

Below is the list of sealing flange types used in high-pressure vessels.

• Weld Neck Flange: Wild neck flanges are equipped with tapered hubs to distribute the stress evenly. Also, standardized welding is used to join the flanges with the vessel’s openings.
• Blind Flange: This flange type is eventually installed at the end portion of a vessel and allows the maintenance crew’s entry to repair, fault check, adjustment works, etc.
• Flange with Ring Type Joint: This flange type is considered high profile as it accommodates metal gaskets developing a highly functional metal-to-metal joint between two flanges.

Core Components of a Pressure Vessel Flange

The following are the core components of a pressure vessel flange.

Flange Face

This is the metal or other material-made circular plate that contacts the gasket’s surface consisting of bolt holes and a bigger hole to connect pipes, the actual opening of the vessel gate, the valve’s opening, etc. Flange face type varies depending on the opening type and vessel’s requirements – flat, raised, or ring-joint type flange. 

Also, there are two types of flange faces, the respective area of the bigger hole is considered face ID, and the area consisting of bolt holes is known as face OD.

• Face’s inner diameter:  This diameter is measured as the opening’s actual diameter allowing the consisting liquid or gasses to pass through easily.
• Face’s outer diameter: Its diameter is determined by considering the bolt holes’ number, size, and structural integrity-related requirements.

Gasket

Gaskets are placed in between the connection of two flanges or surfaces and flange face for one face flanges. Depending on the type of the requirements of the pressure vessel’s properties and the material, the gasket construction material can vary. 

Still, in general, high-performing metal is the construction material for the gasket for flanges installed at a pressure vessel. Gaskets also have two face IDs.

• The inner portion of the gasket’s Dia is actual to the Dia of the vessel’s opening and the flange’s inner dia.
• The outer diameter of a gasket is actual to the face’s outer diameter.

Bolt Holes

Bolt holes reside on the flange face’s extended portion allowing the installation of the bolts. The number, diameter, thickness, and distribution of the holes depend on the flange face’s characteristics and the required pressure tolerance it needs to generate to ensure a tight joint.

Hubs

A flange hub is the extended portion of the flange’s outer side. However, hubs are usually constructed on the blind flange faces to provide better integrity of the overall vessel structure.

Importance of Accurate Calculations for Pressure Vessel’s Flange

The following are the importance of doing accurate calculations for pressure vessel flanges.

Leak Prevention

Pressurized vessels accommodate oil, gas, and chemicals that fall in the hazardous substances category. Any leak in its locking system using flanges leads to water, soil, and air pollution. 

When the world is struggling with a substantially high rate of environmental degradation, any such occurrence is not desirable at all. In this case, accurate flange dimension calculation prevents any leak in the vessel’s opening sections.

Safety

In pressure vessels, the inside substances are kept in a different pressurized condition than the outside environment. So, any dysfunction in its locking, pressure controlling, temperature controlling, etc. systems using flanges can make the vessel act like an explosive. Only accurate flange dimensions and properties can prevent this type of happening.

Structural Integrity

Flanges in the various portions of a pressure vessel are considered subject-critical. Any sort of inaccuracy in its dimension calculation results in the degraded structural integrity of the vessel. 

Here, precise calculation of flange dimension can restrict this ensuring the respective pressurized container is able to handle the outward pressures and inward stresses imposed. The greater structural integrity also ensures greater durability of the vessel.

Compliance with Standards

There are several standardized specifications in the world. Some countries follow standards like ASME or ANSI while other countries follow standards like Din, ANSI, or any other. Depending on the country’s compliances, accurate calculation for pressure vessel flanges is a core requirement.

Operational Efficiency

Accurately calculated flange dimensions offer greater operational efficiency of the pressure vessel. It prevents any pressure and temperature loss while reducing the orientation of leakages. It is also beneficial to reduce the frequent need for maintenance and repair work. Following this, greater operational efficiency triggers higher cost-saving opportunities.

Calculation Factors of a Flange Dimension Selection

With more than 25 years of experience in flange design and manufacturing, experts at Yanhao have identified and would like to share the core calculation factors that need to be considered while designing a flange. 

Here is the list of those influential calculation factors following the compliances of ASME Sec VIII Div 1 and 2 along with ASME B16.5.

Pressure Vessel Operating Measures

Operating pressure, temperature, and accommodating material type in a pressure vessel are the three core measures. These data allow the designers to determine the flange’s construction material, thickness, inner diameter, outer diameter, bolt hole number, bolt hole diameter, bolt hole spacing, etc. 

By incorporating the proposed operating measures accurately, designers can easily determine the flange’s design pressure, temperature, corrosion allowance, outside stress tolerance, and so on.

Pressure Container’s Construction Material

The pressurized container’s construction material is also a crucial factor that influences flange dimension selection. The pressure vessel’s construction material and the flange’s construction must be compatible. 

For instance, SS-made pressurized containers’ flanges must be made of compatible materials, and using construction materials like aluminum is not that logical. However, even if any designer proposes such there is a root cause and the flange’s face thickness will be very high logically.

Unique Requirements of the Buyers

Certainly, a single plant does not go for storing, processing, or supplying oil, gas, chemicals, and any other hazardous material altogether in different containers. It signifies that for each case, different plants have different requirements. Here, even if there is any high-profile plant that embraces all the mentioned material storing, processing, and supplying, they use different pressurized containers. 

Also, the pressure, temperature, corrosive tolerance, etc. are not the same for each plant. Moreover, the outside environments of the plants are not the same due to varying locations. In summation, each plant has its unique requirements which must be considered in the accurate flange dimension calculation process.

Meeting Compliances     

The engineering specification-related regulations in the country where the plant is situated can vary significantly, like the ASME flanges are not the same as ANSI flanges. It demands flange designers consider the respective region’s engineering-related standardized compliance.

Other Factors

The following are some other influential factors in flange design.

• Bolt loading also has a major influence on flange dimension selection.
• There are some additional considerations like corrosion allowance, manufacturing tolerance, fatigue analysis, and thermal stress analysis.

How to Calculate Flange Dimension Using Standardized Codes

Here, we elaborate on how to interpret ASME codes related to a pressure vessel’s flange design in manual calculation. The following is the stepwise guide to do so.

Step 1: Identification of Operational Conditions

In this step, the buyer shares various information with the designer like respective pressurized vessel’s target storing material, operational pressure, operational temperature, outside stress, and so on. Following this, the flange designer finds the respective ASME standards related to the material type, pressure, temperature, etc.

Step 02: Selecting Flange Construction Material

Using the step 1 data, the designer finds the ASME-approved materials listed under ASME B16.5. If the target storing material in the pressurized container is corrosive or the outside environment is highly corrosive, the designer incorporates the table of ASME Section II to find compatible flange material that can resist more than the operational temperature, operational pressure, tolerance allowance, fatigue allowance, etc.

Step 03: Selection of Dimension Specifications

In this case, the designer usually incorporates the temperature-pressure rating table to fix the pressure class. Then, using the identified pressure class, the designer determines the bolt spacing, flange face thickness, bolt hole diameter, torque for the bolt, etc. 

In this case, considering the corrosiveness, cyclic loading, etc. designers incorporate ASME Section VIII Div 1 and other respective sections towards increasing the flange face thickness, increasing bolt size, and so on.

Step 04: Flange Thickness Adjustment

Following step 03’s findings, the flange designers incorporate ASME Section VIII Div 1 Appendix 2 to ensure the selected flange thickness is appropriate. Also, the designers do the calculation of flange thickness using the earlier mentioned formula. By incorporating all these, designers do the flange thickness adjustment.

Step 05: Gasket Design

By considering all the earlier data, the designer goes for gasket design in this step. Here, they use ASME B16.20, B16.21, and other respective to select the gasket type, material, thickness, etc.

Step 06: Flange Face Type Selection

We already know that pressure vessel flanges have three options in terms of flange face type. To select the flange face type, the designer incorporates ASME B16.5 if the flange is under 24 inches and ASME B16.47 if the flange is more than 24 inches large.

Step 07: Final Check Up

Finally, the designer does a final checkup using ASME II, ASME VIII Div 1, ASME B16.5, ASME B16.47, etc.

Example of Flange Dimension Calculation

• Considering the operating conditions with 300°F and 500 psi, flange designers choose compatible materials under the Class 300 flange rating.
• They use the ASME VIII’s formula to select flange dimensions.
• The designer uses ASME B16.5 to determine bolt-related and gasket dimensions under the Class 300 flange rating.
• The ASME 16.5 and ASME B16.47 are used to incorporate the required corrosion allowance.
• Finalization of the design after multiple cross-checking by senior engineers.

Must Consider Factors in the Flange Dimension Calculation

• Flange dia calculation is based on design pressure, design temperature, properties of the pressure vessel containing material, the flow rate of the material in the vessel, and ASME VIII plus B16.5 standards.
• The gasket’s material and thickness selection must count the operating conditions and required performances.
• Bolt size and material selections depend on bolt load and operating condition.
• In the case of cyclic load resistance, flange dimension calculation must consider fatigue analysis.
• If there are possibilities of significant temperature differences, a thermal stress analysis is required.

Conclusion

In conclusion, we hope potential Chinese flange buyers for pressurized vessels will avail themselves of some crucial information regarding flange dimension calculation. Still, even though the manual calculation is necessary, buyers must use software like ANSYS, Autodesk, CAESAR II. Compress, SolidWorks, etc.

We, Yanhao Pipeline Equipment Co., Ltd. in Hebei, have ample capability to provide flange design solutions or expert suggestions on the given design by the buyers. We always try to facilitate our customers with full-fledged packages. 

Finally, we suggest our customers have an in-depth understanding of pressure vessel design principles to perform an accurate and result-oriented calculation for flange design.